The present invention relates generally to a zoom lens system having a field angle of 50.degree. or more at the wide angle end and a coverage from the wide angle to quasi-telephoto region and lends itself well fit for a lens shutter type of still cameras and, more particularly, to a compact zoom lens suitable for use with a camera having a waterproof function in addition to the characteristic mentioned above.
Recently, a number of compact zoom lens systems have been proposed for a lens shutter type of camera lenses which have primarily been provided in the form of zoom lens systems.
For instance, JP-Kokai-56-128911 or JP-Kokai-57-201213 discloses a zoom lens system which comprises two, say, positive and negative, lens units in order from the object side and in which the respective lens units are moved to the image surface side for zooming from the wide angle to telephoto end.
According to JP-Kokai-63-153511 or JP-Kokai-1-252916, on the other hand, a zoom lens system comprising three, say, positive, positive and negative, lens units in order from the object side is proposed. In this zoom lens system, the respective lens units are designed to be moved from the image surface to object side for zooming from the wide angle to telephoto end.
However, the zoom lens systems referred to above are all increased in terms of the total length (i.e., the distance from the first surface of the lens to the film surface), while zooming is carried out from the wide angle to telephoto end by the movement of the first lens unit; in other words, their mechanisms are imperatively complicated so as to incorporate them in waterproof cameras, giving rise to increases in the cost and size of such cameras. Thus, they are unsuitable for use with waterproof cameras.
Among zoom lens systems the total lengths of which are kept invariable during zooming, there is a certain type of zoom lens system which comprises four, say, positive, negative, positive and positive, or alternatively positive, negative, negative and positive, lens units, and is designed to perform zooming by moving the second and third lens units on the optical axis which serve as a variator and a compensator, respectively, as well-known for a long time in the art.
However, this type of zoom lens system is likely to give rise to distortion with an increase in the lens diameter of the first unit, because the first unit is a positive lens so that the entrance pupil is spaced away from the first unit. Therefore, some difficulty is involved in reducing the focal length at the wide angle end, thereby making the field angle wide.
Incidentally, JP-Kokai-56-48607 refers to a zoom lens system comprising four, say, positive, negative, negative and positive, lens units and designed such that the focal length at the wide angle end is reduced to 35 mm. However, this type of zoom lens is again unsuitable for use with a lens shutter camera, because of using many lenses and being increased in terms of the total length and lens diameter as well.
In addition, JP-Kokai-63-70217 comes up with a zoom lens system for a lens shutter camera, in which the first lens unit remains fixed. Problems with this zoom lens system, however, are that its field angle is less than 50.degree. and its zoom ratio is as low as about 1.5.